Many Halogen containing polymers and copolymers are thermoplastic in nature, and, therefore, require heating in order to soften them during fabrication operations such as molding, calendering, and extruding. The heat with which these polymers are processed is usually in the range of 190.degree. C. and is limited in degree and duration by their tendency to deteriorate and decompose. Decomposition of the polymers leads to deterioration of the resultant physical properties of the resin. In addition, a severe darkening in color of the resin results which prohibits its use in lightly colored or transparent articles. This deterioration and decomposition is a major drawback to the use of polyvinyl chloride resins in many applications, and in order to overcome this drawback several products have been proposed as stabilizers for halogen containing vinyl polymers. In general, the stabilizers fall into three main classes , namely, metal soaps, organic chemicals, and organometallic compounds.
It has been characteristic of prior art stabilizing systems that they may not fully prevent the deterioration of resins or polymers, including polyvinyl chloride, during the period when the resins and stabilizer compositions may be heated during processing. During this period, which may be from 5 to 30 minutes or longer, the various ingredients including e.g. pigment, plasticizer, stabilizer, lubricant, etc. may be mixed with the resin and the mixture subjected to the influence of heat and pressure to form a substantially homogeneous mixture. During this period of severe heating (T=190.degree. C. or greater), the resin may deteriorate much more quickly than under normal processing or handling conditions, thereby shortening the period that the resin mixture can be heated.
In injection molding processes, heated resin compositions are injected into molds. In general, viscosity of the resin composition drops as temperature increases; thus, it is possible to reduce the cycle times of injection molding processes by increasing the temperature of the resin composition. Thus, it would be useful to be able to raise the processing temperatures of injection molding and other processes.
Organotin compounds, particularly those having tin bonded to sulfur, such as those which contain a mercapto group, have been found to be an extremely effective type of stabilizer for halogen containing polymers, such as vinyl chloride polymers. These organotin stabilizers are relatively expensive. As a result, it would be desirable to replace at least a portion of the tin present in a given resin formulation with a less expensive material without incurring any significant decrease in heat stability. In addition, it is desirable to enhance the stabilizing properties of less efficient organotin compounds by addition of inexpensive synergistic additive compounds. It is also desirable to obtain longer process times at elevated temperatures and to be able to process resin formulations at higher temperatures.
The use of phosphate materials as primary stabilizers for vinyl polymers was disclosed in U.S. Pat. Nos. 2,218,645 and 2,868,765.
U.S. Pat. No. 2,218,645 teaches that a metal phosphate can be used as a primary stabilizing compound. Lead or alkali-metal salts of oxyacids of boron and phosphorous are used as primary stabilizers. The resultant compositions are claimed to exhibit some color improvement.
U.S. Pat. No. 2,868,765 relates to heat stabilized vinyl chloride resin compositions. A phosphate is uniformly distributed on the surfaces of polymer particles by contacting the polymer particles with an aqueous solution of the phosphate; the treated polymer is then combined with other heat stabilizing compounds.
In general, the prior art disclosure of the utilization of an inorganic phosphate as a stabilizing component of vinyl halide polymer compositions did not recognize the importance of surface area as a determinant of performance. It has now been found that surface area is what enables superior melt viscosity times to be obtained with inorganic phosphates previously thought to be ineffective.
An objective of this invention is to extend the viscosity stability of halogen containing resins, such as polyvinyl chloride, at elevated temperatures, while delaying for as long as possible the development of discoloration.
One aspect of this invention is to reduce the amount of organometallic stabilizer required to achieve a given degree of heat stabilization.
A second aspect of this invention is to improve the performance of less efficient stabilizer compounds by the addition of synergistic additive compounds.
Another objective is to provide novel stabilizer compositions for halogenated vinyl resins and novel processes for stabilizing such resins.
Briefly stated, the invention comprises vinyl halide resins, and, as a stabilizer, an organometallic compound, or a derivative thereof and a finely-divided anhydrous or hydrous sodium or potassium monohydrogen phosphate, lithium dihydrogen phosphate, or an alkali metal pyrophosphate. The addition of the finely divided phosphate results in improved stabilization, namely improved melt viscosity times, of the resultant vinyl resin composition. The new stabilizer composition, which includes the synergistic additive, provides enhanced melt stability in comparison to vinyl compounds using the same organometallic stabilizer compounds.
A preferred halogen-containing thermoplastic resin which exhibits superior melt stabilization is polyvinyl chloride, although also contemplated are other polyvinyl halide polymers, copolymers with vinyl acetate, vinylidene chloride, styrene dialkyl fumarate or maleate, or other alkyl esters of mono-olefinic acids, as well as blends and alloys of vinyl halide polymers with other thermoplastic resins. The polyvinyl halide polymer used is ordinarily and preferably the chloride, although others such as the bromide or flouride may be used. Also included are chlorinated polyvinyl chloride and halogenated polyolefins.